Field of the Invention
The present invention relates to a mist forming method suitable for a fuel injection valve for internal combustion engine (hereinafter, referred to simply as the engine), a fluid injection valve, and a mist forming apparatus.
Background Art
Research and development on engines for vehicle, such as an automobile, are actively conducted to achieve less gas emission when the engine is cold and better fuel consumption by improving combustion characteristics through atomization of a fuel mist.
A fuel injection system of a gasoline engine is classified into a port injection system and an in-cylinder injection system. Three elements important to establish a combustion concept of the in-cylinder injection system are a mist specification (including an injection position), an in-cylinder air flow, and a shape of a combustion chamber. The combustion concept is established only when these elements are well matched. However, an in-cylinder pressure and an in-cylinder air flow vary with an engine speed and a load. An amount of fuel injection and injection timing are adjusted to such a variance and mist characteristics and an in-cylinder mist behavior vary with such an adjustment. Combined with restriction on the layout in an engine room, it is quite difficult to match the three elements always while suppressing adhesion of mist fuel onto an in-cylinder wall surface under various operating conditions.
Meanwhile, as with the three elements to establish the combustion concept of the in-cylinder injection system, it can be said that a mist specification (including an injection position), an intake air flow, and a shape of an intake port are three elements to achieve an optimal injection system in the port injection system. In the port injection system, it is typical for a dual-intake valve engine to inject fuel at the intake valves using corresponding two-direction spray (two-stream spray). In light of this configuration, development is under way to find a mist shape and a mist direction targeting method such that prevent adhesion of mists onto the wall surface of the intake port while enhancing atomization of mists. However, a shape of the intake port and an intake air flow under the influence thereof are not necessarily optimized because of the restriction on the layout in the engine room. Hence, a measure to achieve both of improvement of atomization of mists and a mist shape and an injection direction targeting method is not explicitly disclosed.
In the case of large- and medium-size two-wheel vehicles, many of the vehicles are incapable of injecting fuel at intake valves because of the layout restriction, and it is not certain which injection system concept is most suitable in this case. Expectations are therefore rising for the developments in the future.
Further, carburetors are being replaced by the port injection system in small-size two-wheel vehicles, outboard engines, and utility engines. However, many of these engines are single-intake valve engines and actual circumstances is that fuel is injected at the intake valve in some occasions and off the intake valve in the other occasions by one-direction spray (one-stream spray) also because of the layout problem. It is obvious that a reduction of emission gas and improvement of fuel consumption are demanded further and there is a need for an optimal specification at suppressed system costs.
As has been described, in the case of a mist specification by the two-stream spray, matching in a gasoline engine of the port injection system in the related art is carried out using parameters, specifically, a mist angle and an injection amount distribution image on a cross section perpendicular to an injection direction of each stream of spray, an injection angle of the two-stream spray (sandwiching angle), a content of atomization levels of the mists at a predetermined point.
More specifically, each stream of spray on a cross section perpendicular to the injection direction is of substantially a circular shape or substantially an elliptical shape. A basic specification of an injection amount distribution is substantially a conical solid distribution peaking substantially at or around a center, and atomization is enhanced as the need arises. A level of atomization and a mist angle are correlated with each other. Hence, when a priority is given to one, the other is forced to depend on the circumstances. An injection amount distribution peaks at or around the center because injection directions from respective injection holes are oriented in a direction in which the injection directions concentrate. Hence, a distribution ratio is relatively high at the center. The same can be said with a case of the one-stream spray.
To solve the problems discussed above, various proposals have been made for nozzles or mists as described, for example, in Patent Documents 1 through 6.
[Patent Documents]
Patent Document 1: JP-A-2005-233145
Patent Document 2: JP-A-2004-225598
Patent Document 3: JP-A-2008-169766
Patent Document 4: JP-A-2005-207236
Patent Document 5: JP-A-2007-77809
Patent Document 6: JP-A-2000-104647
None of these proposals, however, describes a measure to achieve both of improvement of atomization of mists and an increase in degree of design freedom for a mist shape, mist pattern, and an injection amount distribution. Hence, none of these proposals provides a guideline to determine an optimal mist specification in actual circumstances where a shape of the intake port and an intake air flow are different from one engine specification to another. Descriptions in this regard in Patent Documents 1 through 6 will be given one by one.
In Patent Document 1, atomization of fuel is promoted by securing an air region between liquid columns from multiple holes to reduce interference of the liquid columns and thereby promoting breakup of the liquid columns into mists. Atomization is promoted by arranging a location of the liquid columns like a part of the cone surface. In practice, it is necessary that most of fuel is turned to liquid threads or in a state close to liquid drops at a position at which interference of the liquid columns occurs. This is because atomization is deteriorated when interference occurs in a state of the liquid columns (see Paragraph [0006] of Patent Document 1). In other words, injection holes are merely located so that interference of the liquid columns occurs at a more downstream position and a measure to control a mist pattern and a shape of a whole mist both made up of a plurality of mists is not disclosed. It is therefore natural that the whole mist tends to spread and a degree of design freedom becomes small. Hence, applications of this proposal are limited by a shape of the intake port and a location of the intake valve.
In Patent Document 2, a gravity center of a fuel injection amount distribution is set closer to the inner sides than mist outline centers of the two-stream spray to form mists aiming at the more inner sides of the both intake valves. Hence, in a case where fuel adhering onto the back surfaces of the intake valves is blown off by an air current, an amount of fuel adhering onto a cylinder bore wall surface becomes minimum.
However, an atomization technique of jets from fuel injection valves is developed considerably in recent years. Accordingly, when an atomization level is put aside, fuel is turned to sufficiently broken up mists by the time the fuel arrives the intake valves. Hence, even during the exhaust stroke injection, there is more mist fuel floating in the intake port than mist fuel adhering onto the intake port and the intake valve due to an air flow in the closed intake port.
There are cases where perfect evaporation and perfect combustion of fuel in the cylinder cannot be expected by only an atomization effect exerted when fuel passes by a channel of the intake valve, and emission of unburnt hydrocarbon (HC) cannot be reduced sufficiently. In particular, temperatures of the intake port and the intake valve are low immediately after a cold start, and it cannot be expected that mist fuel or adhering fuel in these cold places evaporate soon.
Because the emission control is becoming stricter, even when atomization of fuel mists is promoted, it is also necessary to reduce emission of unburnt HC by reducing fuel adhesion onto the intake port and the intake valve. As adhesion of mist fuel onto the intake port and the intake valve is reduced more, a relation of an amount of injection and combustion performance in the corresponding cycles, that is, a relation among an emission gas, fuel consumption, and an output becomes clear. It thus becomes possible to further optimize the overall injection system including controllability.
It is therefore necessary to atomize mists as much as possible to achieve perfect evaporation and perfect combustion. However, Patent Document 2 fails to describe a realization means. Also, the injection amount distribution shown herein schematically shows an injection amount distribution for an image of independent liquid column jets from the respective injection holes when they turn into one jet by interfering with one another moderately, and therefore does not show an injection amount distribution when the liquid column jets from the respective injection holes break up and turn into mists. Accordingly, a shape of the intake port and a location of the intake valves to which the proposal is applicable are uncertain.
In Patent Document 3, atomization is promoted by arranging a location of the injection holes so that mists from the respective injection holes do not interfere with one another and a bias in an injection amount distribution is reduced. However, as with Patent Document 1, interference of mists is merely avoided in Patent Document 3. It is therefore natural that a mist pattern and a shape of a whole mist both made up of a plurality of mists tend to spread and a degree of design freedom becomes small. Hence, applications of this proposal are limited by a shape of the intake port and a location of the intake valves.
It is described that a bias in the injection amount distribution is reduced by locating the injection holes also on the inner side. However, it can be said that a bias in the injection amount of distribution is reduced relatively in comparison with a case where the injection holes are not located on the inner side. A description is not given as to in which manner an injection amount distribution with a reduced bias can be obtained by atomizing mists while avoiding interference among the independent liquid column jets from the respective injection holes. Accordingly, a shape of the intake port and a location of the intake valve to which the proposal is applicable are uncertain.
Patent Document 4 describes that it is preferable to form atomized mists obtained by collision and a lead mist having a high carrying force, so that a fuel mist concentration is made higher in the inner side direction than at the center position of the intake valve by suppressing scattering of the mists by letting the latter pull the former. However, in order to atomize the mists by letting jets collide, it is necessary to set a colliding position at a position shorter than a break length of the jets. In this case, being atomized, the jets (mists) scatter and part of energy of the jets is converted to a surface tension of scattered mist particles by collisions. Hence, a carrying force is reduced.
Even when mists scattered by collision and thereby having a lowered carrying force are pulled by the lead mist injected at the same time and having a high carrying force, timings of behaviors at the tip ends of these mists do not match in time. In the case of a small injection amount with a short injection period, mists scattered by collision are left behind and only the lead mist moves forward.
Besides the one shown in FIG. 4 of Patent Document 4, an attractive swirl developed by the lead mist forms an annular swirl on the outer periphery of the lead mist at a given downstream position in the injection direction determined by a balance of a shearing force between the outer periphery of the lead mist and the atmosphere at the same time. Hence, the scattered mists are caught into the annular swirl and can no longer move downstream in the injection direction.
As has been described, various restriction conditions are necessary for the lead mist to move forward while pulling scattered atomized mists. This proposal is therefore not suitable for the injection system for gasoline engine that is often in an unsteady state during a transient operation. Accordingly, there is a need for a method of increasing a degree of design freedom for a mist pattern and a shape of a whole mist more readily.
In Patent Document 5, a mist pattern is such that allows more fuel to adhere onto an umbrella portion of the intake valve by avoiding the intake valve system, and atomization while the mists pass through the intake valves is used. This proposal, however, has a problem same as that of Patent Document 2.
Patent Document 6 describes that irregularities in a travel direction of mists can be prevented because respective mists are atomized while avoiding interference among them and the respective mists move forward while attracting one another by the Coanda effect. It is, however, difficult to maintain a balance in the fuel direction to let the Coanda effect be exerted so that the respective mists do not spread too much on one hand and to suppress the Coanda effect so that the respective mists do not gather on the other hand even under a static atmospheric condition. Moreover, inside the intake port, there are influences of ambient air pressure and temperature, an intake air flow, a fuel volumetric (weight) flow rate, and a fuel velocity. Hence, it is quite difficult to implement this proposal in the injection system for gasoline engine that is often in an unsteady state during transient operation. In other words, because the Coanda effect does not play a role of actively forming a compact collective mist, a mist shape of a whole mist, a mist pattern, and an injection amount distribution depend on the circumstances.